P. Corcoran

Design of a driver for the Cygnus X-ray source

Cygnus is the prototype of a radiographic x-ray source leveraging existing hardware and designs to drive a rod-pinch diode at 2.25 MV. This high-resolution x-ray source is being developed to support the Sub-Critical Experiments Program (SCE) at the Nevada Test Site (NTS), and as such employs a modular technology that is scaleable to higher voltages and can be readily deployed underground. The diode is driven by three Induction Voltage Adder (IVA) cells from the Sandia SABRE [1] accelerator, threaded by a positive polarity vacuum coax that extends 2 meters to the diode and is designed to operate below electron emission on the anodized outer electrode. The /spl sim/40 ohm diode impedance requires a 40/3/sup 2/ or /spl sim/4.5 ohm source to drive the three IVA cavities in parallel; a convenient impedance for a single water coax. The water coax is designed to function as a two-step impedance transformer as well as a long, passive water cable, accommodating several bends along its length. The latter feature allows independent positioning of the pulsed power driver, IVA and diode x-ray source. The long water coax is driven by a PFL originally developed for Sandias Radiographic Integrated Test Stand (RITS) and a low-inductance commercial Marx charges the single PFL. The accelerator design is a result of a cooperative effort by Titan-PSI and Maxwell (now collectively Titan-PSD) SNLA, LANL, NRL, and Bechtel-Nevada.

Design of a radiographic integrated test stand (RITS) based on a voltage adder, to drive a diode immersed in an high magnetic field

Advanced radiography capabilities can be provided by diodes in which a <1 mm diameter cathode is immersed in a /spl sim/60 T magnetic field and pulsed to /spl sim/16 MV. An electron current of /spl sim/50 kA is constrained by the magnetic field to /spl sim/1 mm at the anode, and produces V 1 krad at 1 m. A 16 MV, 50 ns flat-top voltage adder has been designed that is optimum to develop such diodes. The adder approach is chosen to give a relatively short risetime and low prepulse. It also has the advantage of being re-configurable to provide two 8 MV pulses. The pulser uses standard Marxes, intermediate stores, and water PFLs. Novel features include an oil prepulse switch, an induction cell that is fed from one point, and a blocking network to couple two pulses to one cell. Based on detailed simulations, a design has been completed through detailed drawings and prototype hardware will be tested next year.

Design of an Induction Voltage Adder Based on Gas-Switched Pulse Forming Lines

This paper describes an induction voltage adder (IVA) being designed by Titan Pulse Sciences Division (TPSD) for AWE Aldermaston, UK. This IVA will be used to power the radiography sources in AWEs planned three-axis Hydrodynamic Research Facility (HRF). TPSD will provide the IVAs and AWE will provide the radiographic diodes. The full IVA will in its initial configuration deliver a 14 MV, 110 kA, 50-60 ns pulse first to a large area diode and then to a developmental radiographic diode. The HRF IVA has been designed utilizing two-conductor water dielectric pulse forming lines (pfls) with laser triggered gas switches and can be reconfigured to power a range of radiographic diodes with peak powers from 1.5 to 3.9 TW and peak voltages from 8 to 16.8 MV. LSP simulations of the vacuum region aided by parapotential theory have explored and optimized the coupling of the different IVA configurations to various diodes.

Pulse power performance of the Cygnus 1 and 2 radiographic sources

Cygnus is a two-axis radiographic X-ray facility designed to drive rod-pinch diode loads at 2.25 MV with a spot size of about 1 mm producing 4 Rads at 1 meter. This x-ray source was developed to support the Sub-Critical Experiments Program (SCE) at the Nevada Test Site (NTS) and is distinguished from other commercially available sources by a dramatically reduced spot size for high resolution radiography, higher reliability, and compact size and modularity for greater layout flexibility to fit within the size constraints of its ultimate under-ground site location [D. Weidenheimer et al., June 17-22, 2001]. The facility is composed of two virtually identical machines referred to as Cygnus 1 and Cygnus 2 that incorporate proven pulsed power technology. Each machine employs a Marx generator, pulse forming line (pfl), water coax transmission line, and inductive voltage adder (IVA) that drive a high vacuum rod-pinch diode. The pfl design was originally developed for the radiographic integrated test stand (RITS) [I. D. Smith et al., October 2002] and the IVA cells are from the Sandia SABRE [J. P. Corley et al., 1991] accelerator. The Cygnus 1 machine was constructed and fielded at the Los Alamos National Laboratory to undergo pulsed power component and reliability testing and for use to develop and optimize the rod-pinch diode load [J. R. Smith et al., June 23-28, 2002]. Later, Cygnus 2 was constructed and fielded at Titan-PSD for testing employing the changes and modifications that resulted from of the Cygnus 1 tests. At the time of this writing, Cygnus 2 has undergone testing of the pulsed power components up through the output of the water line where a dummy load was placed. A pulse has not yet been propagated through the water coax to the diode. This paper describes and compares the pulsed power performance of both Cygnus machines up to the output of the water line. The Cygnus testing program is a result of the cooperative effort of Titan PSD, Sandia National Laboratory, Los Alamos National Laboratory, and Bechtel Nevada.

Advances in pulsed power modeling and experimentation on the RITS accelerator

RITS (Radiographic Integrated Test Stand) is planned to be a 12-cell, 16-MV, 150-kA, 70-ns induction voltage adder. A three-cell, 4-MV, 150-kA, 70-ns version (RITS-3) is operating routinely at its specified level at Sandia. Its over-all performance will be described. Advances have been made in understanding and modeling many of the pulsed power features of RITS and several fundamental accelerator design guidelines have been developed. We summarize these. We omit discussion of vacuum power flow and symmetrization, which are the subject of other detailed papers. Subjects include: performance and redesign of the input oil-water diaphram of the pulse forming line (PFL); water switch losses; prepulse measurements at the cell; high voltages breakdowns; and impacts on the induction cell risetime due to the current-symmetrizing azimuthal oil line and the vacuum injection to the magnetically insulated output transmission line.

RITS-3 self-break water switch studies

The radiographic integrated test stand (RITS-3) is a 4-MV, 160-kA, 70-ns inductive voltage adder accelerator at Sandia National Laboratories designed to develop critical understanding of flash radiographic drivers and x-ray sources. It utilizes three pulse forming lines (PFLs) to drive three inductive voltage adder cavities. Each PFL is switched by a fast-pulse-charged, self-breaking, annular water switch. Good synchronization and low loss in the switches is essential to efficient operation of the accelerator. Data from RITS-3 is analyzed with respect to jitter and losses, and compared to computer electric field codes for accuracy. In an attempt to achieve better synchronization, each of the three switch gap sizes was adjusted and electrode geometries were changed to improve performance.

Tests of the first RITS PFL and cell

The Radiographic Integrated Test Stand (RITS) is designed to demonstrate a modular Inductive Voltage Adder accelerator architecture approach to high brightness flash radiography. Multiple, fast (10 ns) rise time, square (70 ns) 1.35-MV, 7.8-Ohm pulses are generated in parallel water dielectric pulse forming and shaping lines, then are added in series with induction cells to form a single high voltage drive pulse. The Metglas-core induction cell has a novel single point feed and tapered azimuthal transmission line to distribute the incoming pulse uniformly around the magnetically insulated vacuum bore of the cell without adding to the pulse rise time. The core induction cells are threaded with an inner vacuum coaxial electrode, which terminates in a high brightness electron beam diode where the bremsstrahlung radiographic source is generated. The first pulse forming line (PFL) and induction cell of the 12 planned for the full RITS facility have been fabricated, assembled and tested at Sandia National Laboratories. The results of these tests are presented and are compared to the design simulation predictions.

Design of a repetitive +30 mv, 4 mj, 9 ns icf reactor driver

Starting in 1984, studies have been made of 15-65 MV, 4 MJ, 9-16 ns ion drivers for the LIBRA ICF reactor. The Helia approach was adopted. Design options and general design considerations are discussed. A recent 30 MV design is described that consists of 16 modules, each of 28 1.3 MV Metglas induction cells, powered by thyratron-switched capacitor banks through magnetic pulse compressors.

PIM-a Blumlein driven IVA machine

The PIM machine has been designed and constructed at AWE to develop IVA technology for flash radiography of hydrodynamic experiments. While it was originally conceived as one module of a ten module, 14 MV, 100 kA machine versions operating at up to 3 MV are of interest to satisfy future radiographic applications at AWE. The IVA architecture will enable these machines to be relatively easily configurable in either negative or positive polarity allowing the diode to be either the self magnetic pinch type already in use at AWE or a rod pinch diode to achieve smaller radiographic spots. A Marx generator drives a 1.7 MV, 10 ohm water Blumlein initiated by twin radial laser triggered switches. The Blumlein has been used to drive either one or two parallel inductive cavities to obtain an output of 1.5 or 3 MV with a current of up to /spl sim/150 kA or >50 kA respectively. Prepulse suppression is provided by a gas prepulse switch in the coaxial oil feed from the Blumlein to the cell or cells. The latest results of the testing of the laser triggering system and the prepulse reduction system will be presented.

Evaluation of pulsed power architectures for active detection

Intense pulsed active detection (IPAD, [1]; also see presentations at this conference by B.V. Weber, et al., D.P Murphy et al., S.B. Swanekamp et al. and J.C. Zier, et al.) has been proposed as a means of detecting contraband fissile material from a distance. In this approach, an intense bremsstrahlung pulse is used to induce photo fission, the products of which are detected. In this work, we report on an initial effort to evaluate the applicability of various pulsed-power architectures to this approach. The electron energy is 12 MeV with an effective electron charge at 12 MeV of 3 mC to 5 mC delivered in <; 100 ms, in either a single pulse or a burst. The eventual goals for the accelerator are compactness, especially short length; relatively light weight; transportability; and ease of setup and operation in the field. We first consider designs that could be constructed in a few years with minimum development. We compare induction voltage adders (IVA), ferrite-core linear induction accelerators (LIA), and linear transformer drivers (LTD). Conceptual point-designs are developed for each approach using essentially demonstrated technology, though the LTD assumes scaling and repackaging. The IVA design was considered for further design development [2]. We then sought approaches that are not demonstrated technology but have promise for achieving substantially less weight and volume. We considered recirculating LI As, auto-accelerators, air-core LI As, dielectric wall accelerators, and vacuum inductive stores with plasma opening switches. A partial pre-conceptual design of a 100 kA, single-pulse air-core LIA suggested that this might be a promising advanced candidate, and it is described here. An evaluation of the other advanced concepts will be published elsewhere [3].